The present invention relates to a method for forming a finely patterned photoresist layer on a substrate or, more particularly, to a method for forming a finely patterned photoresist layer on a substrate in which a layer of the photoresist material is subjected to irradiation in a pattern with actinic rays followed by a dry-process development through exposure to an atmosphere of a plasma gas.
As is well known, the manufacture of semiconductor devices in the semiconductor industry is performed in a number of processing steps, typically including photolithography. The photolithography step is usually performed by use of a photoresist material which is a type of photosensitive composition. In a typical process of photolithography, a silicon wafer is first provided with a thin undercoating film of several hundred nanometers in thickness of aluminum, silicon oxide, silicon nitride, polysilicon and similar material, and then with a coating layer of a photoresist composition. The photoresist layer is then irradiated in a pattern with actinic rays such as ultraviolet light through a photomask having a desired pattern, followed by development with a suitable developer liquid or solution to give a patterned layer of the photoresist. This patterned layer serves as a protective mask for the thin undercoating film in a subsequent etching step whereby the thin undercoating film in the unprotected areas is removed, exposing the substrate surface. Finally, the patterned layer of photoresist is removed to expose the layer of thin undercoating which had not been etched and remains in a pattern corresponding to the photomask pattern.
The above described process of photolithography includes as an essential step development by a wet process using a developer liquid. A problem in wet process development is that the film of photoresist in the areas where the photoresist is not to be removed absorbs the developer liquid and becomes swollen so that the method is not quite satisfactory or practical when the pattern of the photoresist layer formed on the substrate is extremely fine. In addition, the developer liquid which usually is or contains an organic solvent causes serious problems related not only to workers' health but also to environmental pollution.
Accordingly, as a trend in recent years, the wet-process development method which has the inherent problems and disadvantages described above is being replaced by the so-called dry-process development method. In dry-process development, the photoresist layer on the substrate surface is first irradiated in a pattern with actinic rays. It is then subjected to a treatment so as to produce a difference between the irradiated and unirradiated areas of the photoresist layer in its resistance to forming ash when exposed to an atmosphere of a plasma gas. The areas of the photoresist layer where the photoresist layer has less resistance against the plasma gas is thereby removed upon ashing, leaving a patterned layer of the resistant photoresist composition on the substrate surface. Because no developer liquid is used at all, dry-process development is very advantageous when a very fine pattern of the photoresist layer is desired. In addition, problems associated with workers' health and environmental pollution are avoided so that this method is highly valued in the modern semiconductor industry.
Dry-process development hitherto reported includes a method described in Japanese Patents Kokai 57-44143 and 58-60537 according to which the resist layer is irradiated in a pattern with actinic rays, and then subjected to heat treatment in an atmosphere at a temperature in the range from 80.degree. to 180.degree. C. to produce a difference between the irradiated and unirradiated areas in the resistance of the resist layer against the plasma, followed by development by exposure to an atmosphere of a plasma gas.
In the above described method, the upper limit of the temperature for the heat treatment is usually 180.degree. C. because it has been generally understood that the resist layer may be softened and flow when the temperature of the heat treatment exceeds 200.degree. C. Heat treatment at this relatively low temperature takes a relatively long time. Therefore, the substrate such as a silicon wafer is sometimes badly affected. For example, it may become warped as a result of such a prolonged heat treatment. Moreover, the low temperature of the heat treatment can produce only an insufficient difference between the irradiated and unirradiated areas in the resistance of the resist layer against the plasma gas. Accordingly, the residual film thickness of the patterned resist layer formed in this manner is undesirably small, and the method is not suitable for practical use in the formation of a finely patterned resist layer.